Heretofore, fuel injection systems suitable for diesel engines and the like have relied on one or another form of mechanical "jerk" pump to provide the precisely metered and timed quantities of fuel at the extreme pressures and short durations required. Such injection systems vary widely in form and metering principle but virtually all utilize mechanically-driven, cam-actuated, closely-fitted, piston-type pumping elements to accomplish injection during a very small and precisely timed interval of the engine's operating cycle. Since modern diesel engines require the injection of controllably small quantities of fuel into the cylinder at the end of a compression process that is sufficiently severe to produce temperatures exceeding the auto-ignition temperature of the fuel, very high injection pressures are required. The back-pressure of the compressed cylinder charge is only a part of the injection pressure requirement. In addition, an even greater fluid pressure must be established in the injector in order to eject the fluid charge at the rate necessary to meet the cycle timing requirement as well as to attain the velocity and degree of atomization required to penetrate and mix into the dense air charge. Still higher pressures are required in most prior-art injection systems because they utilize inwardly-relieving-type injection nozzles in which the orifices that control admission of the fluid to the combustion chamber are opened and closed by the action of a spring-loaded needle valve that must be lifted hydraulically against such spring pressures. The pressure drop required to lift a needle-type delivery valve and transport the fluid pulse to and through the orifices is particularly burdensome because it is not readily converted to fluid velocity upon which injection quality depends. Moreover, the customary orifices are fixed in size so that fluid velocity achieved varies with the quantity injected. Thus, at low quantities, injection velocities may be too low for good injection quality.
Characteristic of many prior art injectors and injection systems is the loss of fuel metering precision and repeatability as well as injection velocity factors that combine to limit the range of useful engine operations and applications. This is generally known as the "turn-down" limitation. When prior art injectors are operated at less than about fifteen percent of their maximum design delivery (about a 7:1 turn-down ratio), engines using them exhibit undesirable characteristics such as uneven shaft output torque and increased exhaust emissions. These adversities can be avoided in those engine power applications that do not impose excessive variations in loads and speeds requiring a larger range of turn-down injector delivery. However, there are important compression-ignition (diesel) engine applications in which injector turn-down limitations are a handicap. Two examples are given as follows.
Motor vehicles subject their engines to large load and speed variations. In many, a large fraction of on-highway operating time is spent waiting in traffic, decelerating, descending grades and cruising at legal speeds. Under these conditions, the engine is required to run at a very light load or no load at all. Injector turn-down limitations described above account for excessive fuel consumption, exhaust emissions, noise and mechanical harshness.
Diesel engines are capable of operating with a supplementary fuel, in a mode known as dual-fuel operation. This is an established practice that consists of substituting a lower cost vaporous fuel such as natural gas or other suitable fuels for a significant fraction of the petroleum distillate fuel normally delivered to the engine by the fuel injectors. The vaporous supplementary fuel is usually transported into the combustion chamber as a mixture with the intake air stream. The liquid distillate fuel is injected directly into the combustion chamber as in ordinary diesel operation. The flexibility to operate at full power either as a conventional diesel or with a supplementary fuel is highly valued. A further objective is to displace a maximum of the distillate fuel by substituting a lower-cost gas over a wide load range and to the greatest degree possible in dual-fuel operation. The fraction of distillate fuel injected is known as the pilot charge because it is compression ignited to inflame the main fuel charge of gas premixed with air analogous to the use of a spark plug. Proper pilot injection for good dual-fuel operation requires less than five percent of the total fuel. Thus over 20:1 turndown is required of a suitable injection system. However, because of the limited turn-down capabilities of available diesel injectors, these objectives cannot be realized without engine modifications to install an additional set of injectors capable of performing the pilot injection requirements of dual fuel operation, with the original diesel injectors being retained for full diesel operation. This complication and expense detracts from the wider use of the dual-fuel technology, a complication that is attributable to the turn-down limitations of prior art injection systems.
Other draw-backs of the prior art mechanical injection systems stem from the fact that they must be mechanically coupled to the engine shaft. This limitation leads not only to mechanical and/or hydraulic complications but it also possesses other negative ramifications as well. One such disadvantage is that running adjustments in injection timing require complicated and precise mechanisms. Another is that the injection pressures that can be delivered tend to diminish with engine speed which leads to reduced quality of injection, ignition and combustion upon engine lug-down and during idle conditions. Still another disadvantage attributable to pump-line systems is that the length of the high-pressure piping connecting the pump plunger to the injector adversely affects the timing and precision of the injected pulse of fuel due to the elasticity of the fluid and piping which cause complex pressure waves adversely affecting the transient flow of fuel from the injector.
An example of a prior art system is represented by the BKM/Servojet CRIDEC (common rail, intensified, direct electronic control) injection system (see BECK, N. J. et al., "Direct Digital Control of Electronic Injectors, "Society of Automotive Engineers Paper No. 840273). While this prior art system overcomes some of the limitations of the mechanically-driven types, it has several draw-backs of its own. For example, the Servojet fixed-volume, accumulator-type injector relies on fluid compressibility for metering which is subject to wide variations in fluid properties. Applicable fluids differ greatly in their compressibility characteristics which complicates the design and application of the Servojet injector. Further negative ramifications include limited metering range and high rail or supply pressure characteristics, both of which are related to the compressibility phenomenon. High rail pressures are required to produce a significant degree of fluid density change at all and such rail pressures must be varied over a wide range and controlled to a precise degree to manage a limited range of quantity variation. Moreover, inasmuch as the Servojet injector depends on the hydraulic amplification principle, its injection pressure will vary directly with the rail pressure supplied whereupon small injection quantities will be injected at lower pressures and, therefore, lower velocities and longer durations. This characteristic has an adverse effect on engine performance at reduced load. Another adverse characteristic of the Servojet accumulator-type injector is that when large injection quantities are delivered, the rate of injection is very high at the beginning of the process and the rate falls off drastically toward the end. This puts an excessive quantity of fuel into the combustion chamber prior to ignition which occurs only after a certain delay that depends on the Cetane number of the fuel and the engine characteristics. The presence of such excessive quantities of unburned fuel in the combustion chamber prior to ignition produces an excessive rate of pressure rise when ignition does occur. Such pressure rise characteristics cause mechanical roughness known as diesel knock which is accompanied by noise, engine wear and increases in exhaust emissions. Thus, the Servojet injector is disadvantaged at high engine loads as well.
The Servojet CRIDEC injectors are also lacking any internal cooling means. No fuel is circulated within the injector housing whereby the heat of combustion would be effectively isolated from the closely fitted plunger. Such thermal isolation would have to be provided externally.
Still another example of a prior art injection system is the NAVISTAR/CATERPILLAR HEUI two-fluid electrohydraulic injector system (Diesel Progress Engines & Drives, Volume LXI No. 4, April 1995, pp. 30-35). This system utilizes high pressure engine oil as the hydraulic medium to effect a hydraulically-actuated unit injector fed by a low pressure fuel. This system uses a high speed solenoid valve to time the admission of high pressure oil for injection and to time the venting of that oil for metering under the impetus of a return spring. Thus, critical timing events are involved.
One drawback of the HEUI system derives from the extreme timing tolerances, speed and stability required in the solenoid valve which must also handle large instantaneous flow rates of a viscous medium. Another disadvantage is the possibility of fuel contamination of the engine oil. Other disadvantages include the use of return springs that are subject to variation and fatigue and the mechanical complications and exposure to leakage resulting from the use of an additional high-pressure fluid system.
All of the aforementioned prior art injection systems suffer from some or all of the following disadvantages:
1. High mechanical loads and complexity of drive, pumping and injection elements; PA1 2. Decreased injection pressure with decreased engine speed (repetition rate); PA1 3. Decreased injection pressure with decreased engine load (injection quantity); PA1 4. Limited range, precision and repeatability of injected quantity; PA1 5. Difficult control of injection timing during operation; PA1 6. Limited injection pressures due to the limitations in the integrity of hydraulic lines and fittings; PA1 7. Limited injection pressures and quantities due to limitations in supply pumping and control; PA1 8. Loss of injection timing precision due to hydraulic pressure waves and transport volumes in lines and fittings; PA1 9. Diminished injection pressures and range of delivery due to the injector nozzle valves and fixed area orifices used; PA1 10. High supply pressures required; PA1 11. Precise supply pressure control required; PA1 12. High mechanical and/or hydraulic power required; PA1 13. External or supplemental cooling of injector required; PA1 14. Confirmation of injector quantity and timing is absent; PA1 15. Poor matching of injector rate to ignition rate causing increased roughness and exhaust emissions; PA1 16. Lack of injector flexibility to use different fluids; PA1 17. Expensive solenoid construction because injectors require precise, repeatable and very short duration pulse control; PA1 18. Limited injection system repetition rate which, in turn, limits the engine speed and power available without excessive smoke emissions. PA1 1. to use a full-authority electronic control with injector plunger position feedback for the precise management of the timing, quantity, pressure and rate of injection; PA1 2. to enable operation from a constant, low-pressure, common-rail fluid supply with or without pumps; PA1 3. to employ a hydraulically amplified and actuated injector plunger without mechanical drives or lengthy high-pressure piping; PA1 4. to use a constant velocity injection nozzle to maximize delivery velocity at any injection pressure over a wide delivery range; PA1 5. to utilize positive displacement metering with plunger position sensing for precise feedback control of the quantity and timing of delivery; PA1 6. to use a single three-way, cartridge-type, electrically-actuated valve for injection metering and initiation of the injection pulse; PA1 7. to obtain injection quantity modulation without requiring mechanical positioning of plunger or sleeve elements; PA1 8. to provide cooling features within the injector by utilizing the fluid to be injected; PA1 9. to eliminate the need for purging the injector of gases and contaminants that may enter the injector from external sources; PA1 10. to eliminate high pressure hydraulic lines and fittings that can limit the pressure and transient response of injector delivery; PA1 11. to simplify the mechanical and hydraulic characteristics of the injector's construction, installation and operation including integration in an ISO 9000 cartridge assembly; PA1 12. to eliminate, in some applications, the use of springs in injector construction and operation; PA1 13. to increase the range of speed and quantity of injection from a given size of injector without loss of injection pressure or increase of the duration of the injection event; PA1 14. to obtain a more uniform and constant rate of injection at any speed or quantity; PA1 15. to accommodate a wide variety of fluids regardless of their viscosity, density or compressibility; and PA1 16. to provide direct evidence of injector plunger motion whereby the control of injection timing and quantity can be assured with precision and repeatability at any repetition rate and quantity of injection.